Embedded Files
ModC is a completely independent modification. It is a comprehensive rebuild of the amp that goes beyond ModA and ModB while also including and extending the benefits of those mods. If you are starting from a stock amp, ModC is a possible choice if you have some electronics experience. If you’re new to soldering and schematics, I don’t suggest starting here. You’re advised to do ModA or ModB and enjoy the benefits they bring. If you’ve done some electronics work before, read on and see if you’re comfortable with the descriptions and details before starting any of this.
ModC is a completely independent modification. It is a comprehensive rebuild of the amp that goes beyond ModA and ModB while also including and extending the benefits of those mods. If you are starting from a stock amp, ModC is a possible choice if you have some electronics experience. If you’re new to soldering and schematics, I don’t suggest starting here. You’re advised to do ModA or ModB and enjoy the benefits they bring. If you’ve done some electronics work before, read on and see if you’re comfortable with the descriptions and details before starting any of this.
A bit about how we got here… After doing ModB, I was still bothered by the residual hum, hiss, and noise in both the rock and roll inputs. I could have been happy with ModB, but I had too much time on my hands and persistence in asking “why?”. I started with just poking around the amp with a scope and trying to understand where the hum and noise were coming from.
A bit about how we got here… After doing ModB, I was still bothered by the residual hum, hiss, and noise in both the rock and roll inputs. I could have been happy with ModB, but I had too much time on my hands and persistence in asking “why?”. I started with just poking around the amp with a scope and trying to understand where the hum and noise were coming from.
If you look for trouble, you will find it… I found ripple on the B+ lines, ground noise, and signal routing problems. The latter two are not easy to fix since ground noise and signal routing problems usually require significant rewiring of the amp, a potentially messy process with PC boards. I also decided that it was time to get some new caps to replace the “spare parts” I had put in for ModA and ModB. Lastly, the B+ ripple convinced me to put a choke into the power supply, which required drilling new holes (gasp) in the chassis – something I had avoided to this point.
If you look for trouble, you will find it… I found ripple on the B+ lines, ground noise, and signal routing problems. The latter two are not easy to fix since ground noise and signal routing problems usually require significant rewiring of the amp, a potentially messy process with PC boards. I also decided that it was time to get some new caps to replace the “spare parts” I had put in for ModA and ModB. Lastly, the B+ ripple convinced me to put a choke into the power supply, which required drilling new holes (gasp) in the chassis – something I had avoided to this point.
So with the above problems in mind, I made a series of changes that I think were effective. You can listen to the clips below to decide for yourself. The overall schematic image is below (also a PDF at the page bottom). The changes are in green and overlaid on the v.1.4 stock schematic. The notes should be helpful and the frequencies are the RC -3db points in the signal paths. Note that all bass roll-offs are below ~30Hz, which achieves the same goal as ModA, although ModC extends into the driver stages.
So with the above problems in mind, I made a series of changes that I think were effective. You can listen to the clips below to decide for yourself. The overall schematic image is below (also a PDF at the page bottom). The changes are in green and overlaid on the v.1.4 stock schematic. The notes should be helpful and the frequencies are the RC -3db points in the signal paths. Note that all bass roll-offs are below ~30Hz, which achieves the same goal as ModA, although ModC extends into the driver stages.
Power Supply Ripple: First I added the choke - a Hammond 193H. This is relatively straight forward electrically, but the mechanics were a challenge. The stock Power Supply (PS) PCB is mounted to some spacers, but they are too short! The stock filter caps are smashed against the chassis. In fact, if I tightened the PCB mounting screws, the board flexes due to the caps being smashed into the chassis. This is no big deal until you want to mount the choke in the most ideal position, which is directly under the PS PCB. The holes for the choke can be easily drilled from above, but screw heads will stick up and collide with the PCB caps, making it impossible to tighten the PCB mounting screws. To fix this, I got some #6 standoffs to raise the PS PCB about 1/4”. There is now clearance between the choke mounting bolts and the PS caps. Don’t raise the board more than that since it needs clearance between R24 above the PCB and the chassis cover. The choke should be mounted near the power transformer and away from any signal paths (or tubes). I suggest moving in a bit father from the chassis edge since my position produces tight clearance to the cage cover.
Power Supply Ripple: First I added the choke - a Hammond 193H. This is relatively straight forward electrically, but the mechanics were a challenge. The stock Power Supply (PS) PCB is mounted to some spacers, but they are too short! The stock filter caps are smashed against the chassis. In fact, if I tightened the PCB mounting screws, the board flexes due to the caps being smashed into the chassis. This is no big deal until you want to mount the choke in the most ideal position, which is directly under the PS PCB. The holes for the choke can be easily drilled from above, but screw heads will stick up and collide with the PCB caps, making it impossible to tighten the PCB mounting screws. To fix this, I got some #6 standoffs to raise the PS PCB about 1/4”. There is now clearance between the choke mounting bolts and the PS caps. Don’t raise the board more than that since it needs clearance between R24 above the PCB and the chassis cover. The choke should be mounted near the power transformer and away from any signal paths (or tubes). I suggest moving in a bit father from the chassis edge since my position produces tight clearance to the cage cover.
Note that the added 200uf filter cap (also used in ModA) is now rewired to connect to the DB1 bridge output. You have to cut the trace between DB1 and C23. This is a short trace on the PCB and the choke is wired across the cut. The picture below shows the cut trace with the choke and filter cap connections.
Note that the added 200uf filter cap (also used in ModA) is now rewired to connect to the DB1 bridge output. You have to cut the trace between DB1 and C23. This is a short trace on the PCB and the choke is wired across the cut. The picture below shows the cut trace with the choke and filter cap connections.
The additional picture (right) shows the added standoffs to the PS PCB as well as the physical mounting of the filter cap (same mounting of the cap as in ModA).
The additional picture (right) shows the added standoffs to the PS PCB as well as the physical mounting of the filter cap (same mounting of the cap as in ModA).
The schematic notes show the voltage drops across R21 and R24, as well as the currents flowing through them. The PS ripple is also measured before and after the choke. Ripple levels are clearly insignificant once the choke is added so B+ hum is no longer a concern.
The schematic notes show the voltage drops across R21 and R24, as well as the currents flowing through them. The PS ripple is also measured before and after the choke. Ripple levels are clearly insignificant once the choke is added so B+ hum is no longer a concern.
This picture shows the choke above the chassis. Note the right edge of the choke is aligned with the right edge of the PT (0.4” from the chassis edge). I suggest mounting the choke an additional ~½” over to the left to increase clearance to the cage cover (~1” from the chassis edge). Note the choke bolts point upward to minimize the clearance needed under the PS PCB.
This picture shows the choke above the chassis. Note the right edge of the choke is aligned with the right edge of the PT (0.4” from the chassis edge). I suggest mounting the choke an additional ~½” over to the left to increase clearance to the cage cover (~1” from the chassis edge). Note the choke bolts point upward to minimize the clearance needed under the PS PCB.
OK, so after I did the choke mod, I turned on the amp and expected to hear nothing… no hum at all. Imagine my disappointment when I still heard hum coming out of the amp! How is that possible when the measured B+ ripple was a few millivolts? It took a while to figure out where it was coming from and I’ll spare you the details of hair pulling and cussing…
OK, so after I did the choke mod, I turned on the amp and expected to hear nothing… no hum at all. Imagine my disappointment when I still heard hum coming out of the amp! How is that possible when the measured B+ ripple was a few millivolts? It took a while to figure out where it was coming from and I’ll spare you the details of hair pulling and cussing…
Ground and Signal Routing: The story is about grounding and signal routing or lead dress. (See:http://www.geofex.com/article_folders/lead_dress/lead_dress_in_tube_amps.htm ) These issues are not so well solved in this design. This is counter intuitive from my initial impressions that it was built well and a solid design. I still think the amp is solid, but there are some issues that arise from a simplistic grounding convention, and the limits of PCB signal routing.
Ground and Signal Routing: The story is about grounding and signal routing or lead dress. (See:http://www.geofex.com/article_folders/lead_dress/lead_dress_in_tube_amps.htm ) These issues are not so well solved in this design. This is counter intuitive from my initial impressions that it was built well and a solid design. I still think the amp is solid, but there are some issues that arise from a simplistic grounding convention, and the limits of PCB signal routing.
You may not care or follow all this, but I’ll briefly explain. Look at the stock amp layout and wiring. The Power Supply (PS) PCB ground traces are connected to the chassis with two wires from a C5 pin and a corner PC pad. The chassis therefore becomes the only way to connect to the power supply ground – keep this in mind and note what is said about chassis grounds at the end of this web site. Now consider the signal ground. The input jacks are both screwed and therefore connected to the chassis – so signal ground begins at the chassis. There is also a wire connecting the jack ground to the amp PCB (at a pad near the volume control). So, signal ground and power ground meet at the chassis and are routed to the PCB through a single wire.
You may not care or follow all this, but I’ll briefly explain. Look at the stock amp layout and wiring. The Power Supply (PS) PCB ground traces are connected to the chassis with two wires from a C5 pin and a corner PC pad. The chassis therefore becomes the only way to connect to the power supply ground – keep this in mind and note what is said about chassis grounds at the end of this web site. Now consider the signal ground. The input jacks are both screwed and therefore connected to the chassis – so signal ground begins at the chassis. There is also a wire connecting the jack ground to the amp PCB (at a pad near the volume control). So, signal ground and power ground meet at the chassis and are routed to the PCB through a single wire.
Common grounding strategy is to have all grounds meet at one point – the chassis in this case. However, there is a problem with the THD implementation. Power ground and signal ground have to get to the amp circuit on the main PCB. Power and signal both share the wire that connects the jack chassis ground to the main PCB. This means that power ground currents have to flow on the same wire(s) that the input signal ground current has to travel on. That means they share a common path for some distance and ANY power noise voltage developed along that path is essentially added to the input signal – not a good thing. A better approach seen in most modern amps is to isolate power ground paths from signal ground paths and simply connect the two ground lines at some convenient point (usually away from preamp stages) to ensure that both grounds are at the same potential. This approach ensures that there is no shared path over which both power and signal ground currents flow.
Common grounding strategy is to have all grounds meet at one point – the chassis in this case. However, there is a problem with the THD implementation. Power ground and signal ground have to get to the amp circuit on the main PCB. Power and signal both share the wire that connects the jack chassis ground to the main PCB. This means that power ground currents have to flow on the same wire(s) that the input signal ground current has to travel on. That means they share a common path for some distance and ANY power noise voltage developed along that path is essentially added to the input signal – not a good thing. A better approach seen in most modern amps is to isolate power ground paths from signal ground paths and simply connect the two ground lines at some convenient point (usually away from preamp stages) to ensure that both grounds are at the same potential. This approach ensures that there is no shared path over which both power and signal ground currents flow.
Power Ground: Since the PS PCB ground is already tied to the chassis, the main amp PCB power ground path should lead to the chassis or to the PS PCB to provide the power current return path. I opted to rout the main PCB power ground connection to the PS PCB. This was done with a single heavy wire running from C19 on the PS PCB through an existing grommet opening to the ground plane on main PCB. I used the ground pad for R30 (which is unused) on the main PCB. This connection is at the V2 and power tube end of the main PCB to minimize its impact on the signal ground connections that we will make near V1. See the photo below that shows this wire (white) connecting the two PCBs.
Power Ground: Since the PS PCB ground is already tied to the chassis, the main amp PCB power ground path should lead to the chassis or to the PS PCB to provide the power current return path. I opted to rout the main PCB power ground connection to the PS PCB. This was done with a single heavy wire running from C19 on the PS PCB through an existing grommet opening to the ground plane on main PCB. I used the ground pad for R30 (which is unused) on the main PCB. This connection is at the V2 and power tube end of the main PCB to minimize its impact on the signal ground connections that we will make near V1. See the photo below that shows this wire (white) connecting the two PCBs.
Rock Jack: Signal ground can now be isolated from the power ground (and chassis) to minimize noise presented to the input stages. This means we need to isolate the input jacks from chassis ground. The stock jacks can be replaced by isolated jacks such as the Neutrik jacks. I used the stock jacks with added isolation washers. In either case, you’re likely to need to expand the jack holes to ~0.5” diameter. There is plenty of clearance to do this, but be careful with the faceplate to avoid scratches or bending during drilling. You may be able to find some isolated jacks that fit the stock holes.
Rock Jack: Signal ground can now be isolated from the power ground (and chassis) to minimize noise presented to the input stages. This means we need to isolate the input jacks from chassis ground. The stock jacks can be replaced by isolated jacks such as the Neutrik jacks. I used the stock jacks with added isolation washers. In either case, you’re likely to need to expand the jack holes to ~0.5” diameter. There is plenty of clearance to do this, but be careful with the faceplate to avoid scratches or bending during drilling. You may be able to find some isolated jacks that fit the stock holes.
Regardless, once the jacks are isolated, wire R1 directly on Rock input jack. You can remove R1 from the PCB and reuse it. The PCB R1 connections and traces will not be used anymore since a shielded cable will bypass the PCB traces between J1 and V1. Mount R2 directly on the V1 socket between Pin2 and the center pin. That center pin is unused so it makes a convenient tie point. Now run a shielded cable from J1 signal and ground to V1 center pin (R2) and a ground point near R5.
Regardless, once the jacks are isolated, wire R1 directly on Rock input jack. You can remove R1 from the PCB and reuse it. The PCB R1 connections and traces will not be used anymore since a shielded cable will bypass the PCB traces between J1 and V1. Mount R2 directly on the V1 socket between Pin2 and the center pin. That center pin is unused so it makes a convenient tie point. Now run a shielded cable from J1 signal and ground to V1 center pin (R2) and a ground point near R5.
Note that you now have a signal and ground path between the Rock jack and V1 that has no power current sharing any of the path, and the signal path is shielded to reduce stray pickup. The picture below shows the Rock input jack, with R1 mounted to it and the shielded cable running to V1. Note that the Rock Tip and Ground connections on the PCB (visible in the image) are no longer needed or used. Remove the wires to these pads.
Note that you now have a signal and ground path between the Rock jack and V1 that has no power current sharing any of the path, and the signal path is shielded to reduce stray pickup. The picture below shows the Rock input jack, with R1 mounted to it and the shielded cable running to V1. Note that the Rock Tip and Ground connections on the PCB (visible in the image) are no longer needed or used. Remove the wires to these pads.
The second image shows the connections to V1 and R2. Note also that the wire connecting V1 pin2 to the PCB is removed. The ModC schematic also shows that C1, the 22pf cap mounted on the V1 socket is replaced with a 5pf cap. It’s best to do this now before connecting the shielded cable. The shielded cable replaces the PCB traces to V1 pin2. That means R2 on the PCB is unused. You can remove it and reuse it on V1, mounted as shown in the picture.
The second image shows the connections to V1 and R2. Note also that the wire connecting V1 pin2 to the PCB is removed. The ModC schematic also shows that C1, the 22pf cap mounted on the V1 socket is replaced with a 5pf cap. It’s best to do this now before connecting the shielded cable. The shielded cable replaces the PCB traces to V1 pin2. That means R2 on the PCB is unused. You can remove it and reuse it on V1, mounted as shown in the picture.
Ignore the second shielded cable shown in the photo for now – we’ll get to that in the next section. The shielded conductor connects to the center pin holding R2. The cable shield is extended and insulated to connect to the PCB ground plane near R5. (You could also connect to the ground side of R5.) Lightly scrape some of the green insulation from the ground plane at the point you want to expose and when you have a few sq-mm of shiny copper area exposed, solder directly to that.
Ignore the second shielded cable shown in the photo for now – we’ll get to that in the next section. The shielded conductor connects to the center pin holding R2. The cable shield is extended and insulated to connect to the PCB ground plane near R5. (You could also connect to the ground side of R5.) Lightly scrape some of the green insulation from the ground plane at the point you want to expose and when you have a few sq-mm of shiny copper area exposed, solder directly to that.
This picture shows an added capacitor also connected to the same ground point. Ignore the cap for now, I’ll get to that in a later section. At this point we have rerouted the signal and ground paths from the Rock input jack to the V1 preamp stage. Now we’ll do the same for the Roll input.
This picture shows an added capacitor also connected to the same ground point. Ignore the cap for now, I’ll get to that in a later section. At this point we have rerouted the signal and ground paths from the Rock input jack to the V1 preamp stage. Now we’ll do the same for the Roll input.
Roll Jack: Isolate the Roll input jack and tie its ground pin to the Rock jack’s ground pin. The Roll Finger is still tied to the PCB pad as before. Remove the connection from the Roll tip to the PCB. We are not going to use the R4 - C6 network or traces on the PCB. The traces meander all over the PCB and pick up excessive noise. Instead, we wire the Roll jack tip to a mini switch (SW6) mounted near the volume control, as shown in the image below.
Roll Jack: Isolate the Roll input jack and tie its ground pin to the Rock jack’s ground pin. The Roll Finger is still tied to the PCB pad as before. Remove the connection from the Roll tip to the PCB. We are not going to use the R4 - C6 network or traces on the PCB. The traces meander all over the PCB and pick up excessive noise. Instead, we wire the Roll jack tip to a mini switch (SW6) mounted near the volume control, as shown in the image below.
That connection from Roll jack to SW6 is short and the connection from there to the volume control is short, which helps reduce stray noise. SW6 is added to replace the fixed R4-C6 network in a way that provides a stock boost at 335Hz, a new boost setting at 670Hz (an octave higher), and a slight boost at 14Khz (essentially no boost). Other values can be used for this network or it can be hardwired without a switch. The important thing to do is get the Roll tip signal to the volume control (pin1) with short wires off the PCB.
That connection from Roll jack to SW6 is short and the connection from there to the volume control is short, which helps reduce stray noise. SW6 is added to replace the fixed R4-C6 network in a way that provides a stock boost at 335Hz, a new boost setting at 670Hz (an octave higher), and a slight boost at 14Khz (essentially no boost). Other values can be used for this network or it can be hardwired without a switch. The important thing to do is get the Roll tip signal to the volume control (pin1) with short wires off the PCB.
My preference is to remove C7. If you want a bright cap, add it on the volume control lugs. If you use a switch for SW6, be careful drilling the hole as there barely enough room between the volume control and the steel plate shield that gets screwed in place later. I suggest building the SW6 assembly with it's R's and C's first (see picture above), then mounting the whole thing inside the amp and connecting the input and output lines. The connection from the volume control to V1 is made by another shielded cable from the volume control wiper and ground pins to V1 pin7 and the ground end of R9 (see image above). Note that the wire between V1 pin7 and the PCB is removed. Also note that Cz, a 5pf cap is added directly on the V1 pins. This cap is a minimal value and its purpose is discussed more in ModB.
My preference is to remove C7. If you want a bright cap, add it on the volume control lugs. If you use a switch for SW6, be careful drilling the hole as there barely enough room between the volume control and the steel plate shield that gets screwed in place later. I suggest building the SW6 assembly with it's R's and C's first (see picture above), then mounting the whole thing inside the amp and connecting the input and output lines. The connection from the volume control to V1 is made by another shielded cable from the volume control wiper and ground pins to V1 pin7 and the ground end of R9 (see image above). Note that the wire between V1 pin7 and the PCB is removed. Also note that Cz, a 5pf cap is added directly on the V1 pins. This cap is a minimal value and its purpose is discussed more in ModB.
Now both V1 triodes have clean signals routed to their grids with isolated ground paths and shielded leads. I suggest creating a mechanical strain relief for the shielded cables to minimize the chance of breaking a connection. See the V1 image above (in the Rock section) for a strap example that holds these cables. We now have clean signals for the tubes to amplify and a clean power/ground environment for the tubes to do what they do best… make beautiful sound. The next photo shows the outside of the amp with isolated input jacks (note the fiber washers) and the location of SW6.
Now both V1 triodes have clean signals routed to their grids with isolated ground paths and shielded leads. I suggest creating a mechanical strain relief for the shielded cables to minimize the chance of breaking a connection. See the V1 image above (in the Rock section) for a strap example that holds these cables. We now have clean signals for the tubes to amplify and a clean power/ground environment for the tubes to do what they do best… make beautiful sound. The next photo shows the outside of the amp with isolated input jacks (note the fiber washers) and the location of SW6.
Bass Response Extension: As in ModA, the ModC schematic shows new cap values in many locations to ensure the low-freq response of the amp is preserved. My ModA cap values were pretty much set by what I had on hand in spare parts. For ModC, I computed the values needed and purchased good poly caps that would fit into the PCB. I also extended the bass range changes into the tone and driver sections of the amp. Here is a list of mod caps and their new values.
Bass Response Extension: As in ModA, the ModC schematic shows new cap values in many locations to ensure the low-freq response of the amp is preserved. My ModA cap values were pretty much set by what I had on hand in spare parts. For ModC, I computed the values needed and purchased good poly caps that would fit into the PCB. I also extended the bass range changes into the tone and driver sections of the amp. Here is a list of mod caps and their new values.
C1 becomes 5pf 500v mica
C1 becomes 5pf 500v mica
Cz is added as 5pf 500v mica
Cz is added as 5pf 500v mica
C2 becomes 4.7nf 630v poly
C2 becomes 4.7nf 630v poly
C3 becomes 4.7uf 100v poly
C3 becomes 4.7uf 100v poly
C6-R4 network is replaced by SW6 assembly with (2) 240pf 100v mica caps and 1M and 47K resistors (1/4w).
C6-R4 network is replaced by SW6 assembly with (2) 240pf 100v mica caps and 1M and 47K resistors (1/4w).
C7 is removed and not used
C7 is removed and not used
C8 becomes 10uf 100v poly
C8 becomes 10uf 100v poly
C9 becomes 47nf 630v poly
C9 becomes 47nf 630v poly
C15 becomes 47nf 630v poly
C15 becomes 47nf 630v poly
C20 remains with added 100uf 63v electrolytic in parallel
C20 remains with added 100uf 63v electrolytic in parallel
200uf 500v electrolytic is added to the DB1 output (input of the choke)
200uf 500v electrolytic is added to the DB1 output (input of the choke)
Driver grid resistor: V2 is the final driver stage and it’s driven quite hard on its grid pin7. To minimize grid currents during hard clips, I added a 2.7K resistor in series with the grid. The resistor simply replaces the wire between the PCB and the tube socket. Insulate the resistor and its leads with some tubing as a precaution. This serves the same function as R25 does for V3. A chassis photo below shows this resistor.
Driver grid resistor: V2 is the final driver stage and it’s driven quite hard on its grid pin7. To minimize grid currents during hard clips, I added a 2.7K resistor in series with the grid. The resistor simply replaces the wire between the PCB and the tube socket. Insulate the resistor and its leads with some tubing as a precaution. This serves the same function as R25 does for V3. A chassis photo below shows this resistor.
Local Filter Caps: Although the schematic shows C4 and C5 as “near” V1, these caps are physically on the PS PCB. Long wires and PC traces route the B+ current to the V1 and V2 circuits. These lines pick up noise and cross coupling between the triode stages. It’s common practice to provide some local bypass caps to keep the B+ lines clean, but there are none in the stock design. It’s simple to add a few, and in fact we can reuse the original caps we replaced for added bass response. C9 and C15 are 22nf 630v caps. These are big enough for this purpose, although bigger caps can’t hurt as long as they are rated at 630v or more. The idea is to ensure clean B+ to each triode stage, so we add a cap to the B+ side of both R3 and R8. R14 should also get a cap to ensure clean B+ for the driver stage, so I added a new cap (0.22uf 630v poly). All these caps need a ground connection, so I created a local ground plane connection for them. The picture below shows the final PCB with these three caps identified. Other arrangements are possible to physically mount these caps, but lead length should be minimal.
Local Filter Caps: Although the schematic shows C4 and C5 as “near” V1, these caps are physically on the PS PCB. Long wires and PC traces route the B+ current to the V1 and V2 circuits. These lines pick up noise and cross coupling between the triode stages. It’s common practice to provide some local bypass caps to keep the B+ lines clean, but there are none in the stock design. It’s simple to add a few, and in fact we can reuse the original caps we replaced for added bass response. C9 and C15 are 22nf 630v caps. These are big enough for this purpose, although bigger caps can’t hurt as long as they are rated at 630v or more. The idea is to ensure clean B+ to each triode stage, so we add a cap to the B+ side of both R3 and R8. R14 should also get a cap to ensure clean B+ for the driver stage, so I added a new cap (0.22uf 630v poly). All these caps need a ground connection, so I created a local ground plane connection for them. The picture below shows the final PCB with these three caps identified. Other arrangements are possible to physically mount these caps, but lead length should be minimal.
Resistor elevation: Elevate R15 and R17 slightly from the PCB to ensure isolation from the B+ lines that run under then. A web site reported this as a source of amp failure. (http://www.chambonino.com/work/miscguitar/misc8.html) It seems prudent to do this since it costs nothing while potentially sparing you a nasty amp blowout.
Resistor elevation: Elevate R15 and R17 slightly from the PCB to ensure isolation from the B+ lines that run under then. A web site reported this as a source of amp failure. (http://www.chambonino.com/work/miscguitar/misc8.html) It seems prudent to do this since it costs nothing while potentially sparing you a nasty amp blowout.
In summary, we’ve mainly cleaned up the B+ lines, isolated the ground paths for power and signals, rerouted and shielded the low-level input lines, and recapped the whole signal path to maintain bass response. These changes may seem minor, especially since the amp worked OK to begin with, but the accumulated effects of all these changes make a very significant difference in the sound (IMO). The results are best appreciated by hearing and playing the final amp. The clips below are the best I can offer unless you’re in the neighborhood. My reaction to the new sound is (a) I really like it and (b) surprise since I started ModC to reduce noise, not to alter the tone. This process also renewed my appreciation for the hand-wired “art” of amp building. A PCB is great for low-cost, consistency, and simplicity, but it can constrain signal routing options unless care is taken. Hand-wiring, since it easily includes shielded cables, may be simplest for squeezing the most out of those tubes.
In summary, we’ve mainly cleaned up the B+ lines, isolated the ground paths for power and signals, rerouted and shielded the low-level input lines, and recapped the whole signal path to maintain bass response. These changes may seem minor, especially since the amp worked OK to begin with, but the accumulated effects of all these changes make a very significant difference in the sound (IMO). The results are best appreciated by hearing and playing the final amp. The clips below are the best I can offer unless you’re in the neighborhood. My reaction to the new sound is (a) I really like it and (b) surprise since I started ModC to reduce noise, not to alter the tone. This process also renewed my appreciation for the hand-wired “art” of amp building. A PCB is great for low-cost, consistency, and simplicity, but it can constrain signal routing options unless care is taken. Hand-wiring, since it easily includes shielded cables, may be simplest for squeezing the most out of those tubes.
Whether you just read this whole thing, or actually did the mods, I would appreciate hearing your thoughts and reactions. I started a new section for posting the info and pics people send me about their own ideas and mods for the Univalve.
Whether you just read this whole thing, or actually did the mods, I would appreciate hearing your thoughts and reactions. I started a new section for posting the info and pics people send me about their own ideas and mods for the Univalve.
Audio Clips: I'm playing the Vox Virage with both PUs (in P-90 mode). There is some PU switching in both clips for a sense of the tonal range. The guitar feeds the TLA set for flat response with just a touch of compression. The TLA drives the GSP, which is also set to flat response, and no modeling enabled, but with some tremolo and Hall Reverb. The GSP output feeds the THD inputs and the THD line output goes to an M-Audio FastTrack Pro that is recorded by Audacity 1.3. Sorry about a bit of clipping at a few points, but it was too early for the recording engineer to have his act together. :-) Amp tubes are (2x) 12ax7s and a 6550 output. Noise reduction is off. I left a few seconds of quiet in both clips to give a sense of how quiet the amp is now.
Audio Clips: I'm playing the Vox Virage with both PUs (in P-90 mode). There is some PU switching in both clips for a sense of the tonal range. The guitar feeds the TLA set for flat response with just a touch of compression. The TLA drives the GSP, which is also set to flat response, and no modeling enabled, but with some tremolo and Hall Reverb. The GSP output feeds the THD inputs and the THD line output goes to an M-Audio FastTrack Pro that is recorded by Audacity 1.3. Sorry about a bit of clipping at a few points, but it was too early for the recording engineer to have his act together. :-) Amp tubes are (2x) 12ax7s and a 6550 output. Noise reduction is off. I left a few seconds of quiet in both clips to give a sense of how quiet the amp is now.
The Rock clip amp settings are HiV, 11am volume, 2pm bass/treble, and max attitude. SW6 was set to "normal".
The Rock clip amp settings are HiV, 11am volume, 2pm bass/treble, and max attitude. SW6 was set to "normal".
Late in the clip, the Volume gets upped to about 2pm.
Late in the clip, the Volume gets upped to about 2pm.
The Roll clip settings are HiV, noon volume, 2pm bass/treble/attitude. SW6 was set to "normal". I intentionally hung on a few chords and notes to let them ring and decay. The tone is not "hi-fi" clean, (it's not supposed to be) but it has a nice clear musical bell-like quality. I love the way the notes attack and ring and chime with a full-body quality to them. I just can't get this from the Fender or Vox amps. They lack some aspect of the tone no matter what I do with their tone controls.
The Roll clip settings are HiV, noon volume, 2pm bass/treble/attitude. SW6 was set to "normal". I intentionally hung on a few chords and notes to let them ring and decay. The tone is not "hi-fi" clean, (it's not supposed to be) but it has a nice clear musical bell-like quality. I love the way the notes attack and ring and chime with a full-body quality to them. I just can't get this from the Fender or Vox amps. They lack some aspect of the tone no matter what I do with their tone controls.
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